The invention relates generally to direct-sequence spread-spectrum (DSSS) communication systems.
Spread-spectrum is a transmission technique according to which the bandwidth of the transmitted signal is intentionally increased (usually substantially) beyond the minimum bandwidth that is necessary to send the information. The "spreading" of the bandwidth is accomplished by a code or a spreading signal that is independent of the data. At the receiver, the same code or spreading signal is used to despread the received signal to recover the original signal.
One reason for wanting to increase the signal bandwidth is that the power that is required to maintain a given channel capacity decreases as the bandwidth of the channel increases. In addition, the signal to noise ratio of the transmitted signal becomes larger due to its being spread across a larger bandwidth and thus the transmitted signal is less susceptible to co-channel interference or hostile jamming signals.
Direct-sequence spread-spectrum, which is a common spread-spectrum technique, is a double sideband, suppressed carrier modulation technique which uses a code that has a polar waveform. In other words, the spread spectrum signal, s(t) is generated from the message r(t) as follows: EQU s(t)=A.sub.c .multidot.r(t).multidot.c(t),
where c(t) is the spreading signal having a polar waveform.
In a typical DSSS communication system, such as a system that conforms to the IEEE 802.11 standard, the transmit signal is spread by a finite length sequence (e.g. a barker code of length 11) and then modulated by a differential QPSK (Quadrature-Phase-Shift-Keyed) modulator. At the receiver end, the reverse process is performed. That is, the received signal is first despread by the same sequence and then demodulated by a corresponding differential QPSK detector. The signal can be either despread by a linear correlator for simplicity or by a matched filter for fast acquisition purpose. Despreading by a matched filter is usually utilized for a short preamble system like IEEE 802.11 due to the need for fast acquisition.
The operation of a match filter can be formulated as follows: ##EQU1## where m(t) denotes the match filter output, L is the spreading code length, r(t) is the receive (baseband)
signal, T.sub.c is the chip time period, and ##EQU2## represents the spreading code.
A conventional spreading and despreading scheme is illustrated in FIG. 1. In this system, a received spread spectrum signal 10 is mixed with a locally generated reference signal to generate a corresponding baseband signal. A match filter 12, such as the one described above, despreads the baseband signal. A power detector 14 monitors the output of match filter 12. If instantaneous receive signal power exceeds a preset threshold at a specific (sampling) time epoch, the match filter output is sampled and held in a peak hold circuit 16 until a differential demodulator 18 differentially demodulates the signal and a QPSK decision circuit 20 extracts the desired source information. Timing recovery is performed based on the occurrences of signal peaks. The recovered timing information is then applied to other signal processing blocks that require the timing information to correctly perform their functions (e.g. the frequency discriminator, the differential demodulator, the QPSK decision module, etc.). A frequency discriminator 22 determines the frequency error and controls a local oscillator 24 (also labeled L.O.) to compensate for the frequency error that is incurred during signal propagation. Typically, the phase and frequency errors come from the time-variant characteristics of the propagation channel and the mobility of mobile unit.
Other conventional techniques which apply a match filter in a DSSS communication system are described in U.S. Pat No. 5,377,226; U.S. Pat. No. 5,343,496; U.S. Pat. No. 5,204,875; U.S. Pat. No. 5,181,225; and U.S. Pat. No. 5,363,403. In general, in such other techniques, the signal at the match filter's output is first passed through a QPSK slicer (i.e., it is hard decisioned), and then, the sliced signal is differentially demodulated to recover the transmitted information.
Conventional methods have several implied characteristics. First, some circuits, which might be either optimal or sub-optimal, are necessary to determine instants at which to sample the match filter output. Typically, only a power peak within a symbol period at the match filter output is processed. For example, the match filter output signal is sampled at the time epoch corresponding to the peak and held until decision is made. All other peaks and their associated match filter output samples that occur within the same symbol period are discarded. Secondly, symbol timing information is determined and performed based solely upon the selected peaks. Third, frequency and phase errors are detected and compensated for based on the outputs of the differential demodulator.